High-pressure discharge lamp

ABSTRACT

The invention relates to a high-pressure discharge lamp  1  comprising at least one burner  2  having a discharge space, and comprising two electrodes extending in the discharge space, a gas filling in the discharge space comprising at least an inert gas and a metal halide mixture, a tubular outer bulb  3  having two ends, the burner  2  being attached, at least at one end, to the outer bulb  3 , said outer bulb  3  comprising at least one light-absorbing means  5  and at least one interference filter  6 , and an interference filter  4  being arranged on or in at least a part of the burner  2.

The present invention relates to a high-pressure discharge lampcomprising a burner having a discharge space, and comprising twoelectrodes extending in said discharge space, a gas filling in thedischarge space that contains at least an inert gas and a metal halidemixture, and comprising a tubular outer bulb having two ends, the burnerbeing attached, at least at one end, to said outer bulb.

This high-pressure discharge lamp comprising an outer bulb can suitablybe used for general lighting purposes. Said high-pressure discharge lampis particularly suited for use as a headlight lamp in a vehicle, such asa motorcar.

A discharge lamp of said type comprising an inner bulb and an outer bulbis known from EP 0 964 431 B1. The discharge lamp described thereincomprises an arc tube having a light-emitting region provided with anelectrode pair, and an outer tube surrounding said light-emitting regionand being at least partly fused to the arc tube, said outer tubecomprising silicon dioxide (SiO₂) as the main constituent.

WO 01/24224 A1 discloses a halogen lamp having light-absorbingproperties. This lamp comprises, in this connection, a light-absorbingmeans and an interference filter, which interference filter is arrangedon the outer surface of the lamp bulb, and the absorbing layer must besituated, for functional reasons, between the lamp bulb and theinterference filter. Due to said combination of a light-absorbing meansand an interference filter, which both operate, in particular, in thewavelength range of 570 to 620 nm, the emitted visible light contains asuper proportional fraction of amber-colored light.

Transferring the above-mentioned solution to the discharge vessel of ahigh-pressure discharge lamp is impossible, particularly with regard tothe higher operating temperature of this lamp.

A drawback of the discharge lamps in accordance with the state of theart is that the color point of the emitted light in accordance with theCIE 1931 diagram does not lie in the so-termed “front fog” range inaccordance with ECE R99. In addition, the discharge lamps in accordancewith the state of the art often exhibit a poor light output (ln/Watt).

It is an object of the present invention to provide a high-pressuredischarge lamp having a color temperature of the emitted light below3000 K, the color point of the emitted light lying within the “frontfog” range in accordance with ECE R99, and the lamp exhibiting a lightoutput of at least 60 lm/Watt.

In the case of the yellow light in accordance with the invention, thecolor characteristics must lie within the range defined by the followinglimiting values: towards red y > 0.138 + 0.580 x towards green y < 1.29x − 0.100 towards white y > −x + 0.940 and y > 0.440 towards thespectrum locus y < −x + 0.992.

If the term “front fog” is used within the context of the invention, itis to be taken to mean the yellow color in accordance with the CIE 1931of the yellow range of the corresponding diagram.

The object in accordance with the invention is achieved by ahigh-pressure discharge lamp whose outer bulb comprises at least onelight-absorbing means and at least one interference filter, and aninterference filter is arranged on or in at least a part of the burner.

An ionizing gas filling in accordance with the invention comprises atleast an inert gas as well as mercury in the range of 0 mg to 10 mg.

A lower color temperature in the “front fog” range enables the vision ofthe driver of the vehicle to be improved, in particular, under badweather conditions such as fog. The visible yellow light emitted bymeans of the high-pressure discharge lamp in accordance with theinvention is better adapted to the natural sensitivity of the human eye,so that overstressing in this connection and the associated fatigue areprecluded. As a result, in particular a higher traffic safety isachieved.

In addition, the high-pressure discharge lamp in accordance with theinvention also enables a good light output (lm/Watt) to be attained. Forexample, the light output of the light emitted by such a high-pressuredischarge lamp is at least 60 lm/Watt, preferably ≧70 lm/Watt. Moreover,by means of the high-pressure discharge lamps in accordance with theinvention, also light outputs ≧80 lm/Watt and higher can be achieved.

Advantageous embodiments of the high-pressure discharge lamp inaccordance with the invention are apparent from claims 2 through 12.

It is preferred that at least one interference filter is provided on theouter surface of the burner. By thus providing it in the direct vicinityof the light source, the desired constituents of the emitted light areeffectively filtered. Constituents which are not desired are reflectedas much as possible in the discharge space and at least partly convertedback into light. This leads overall to a hotter absorbing area of thedischarge space, i.e. more light is emitted from the discharge space,and the otherwise customary, significant decrease of the light output isnot observed. It is thus achieved that the desired yellow light having asufficient light intensity is made available.

It is particularly preferred that a light-absorbing means is provided onthe inner surface of the outer bulb, and a further light-absorbing meansis provided between the outer surface of the outer bulb and theinterference filter. By virtue thereof, an effective reduction of theremaining, undesirable constituents of the light is achieved. Within thescope of industrial manufacturing it is technologically simple toprovide an identical, light-absorbing means on the inside and theoutside of the outer bulb, in this case applying a coating to the outerbulb.

In accordance with a preferred embodiment, at least at the surfaces ofthe areas that are used to attach the burner to the outer bulb, nolight-absorbing means and/or interference filters are provided. Byvirtue thereof, the high-pressure discharge lamp can be mass-produced ina technologically simple way.

In a further modification of said embodiment it is preferred that thelight transmittance of the interference filter arranged on the burner,and of the interference filter arranged on the outer bulb, with regardto the wavelength range of 600 to 800 nm, is >90% for both.

It is further preferred that the light transmittance of thelight-absorbing means with regard to the wavelength range of 600 to 800nm ranges between 70 and 90%.

In respect of an effective manufacture and functionality of theinterference filter, it is preferred that the thickness of each filterranges between 800 and 2800 nm.

The interference filter usually has a multilayer structure, saidmultilayer structure being such that a layer having a higher refractiveindex alternates with a layer having a lower refractive index. In thisconnection, the layer having the lower refractive index efficaciouslycomprises predominantly SiO₂ and the second layer comprises a materialhaving a higher refractive index than SiO₂. Said second layer ispreferably composed of a material selected from the group consisting oftitanium oxide, tantalum oxide, niobium oxide, hafnium oxide, siliconnitride, very preferably zirconium oxide (ZrO₂), or a mixture of thesematerials. The materials used for the interference filter aretemperature resistant up to at least 900° C.

The layer thickness of the light-absorbing means preferably lies in arange between 5 nm and 10,000 nm.

The light-absorbing means preferably comprises inorganic pigments thatabsorb part of the visible light. The average diameter of the inorganicpigments should usually be smaller than or equal to 100 nm to ensure thedesired light transmission of the layer and preclude light scattering asmuch as possible.

It is also preferred that the inorganic pigment is composed of amaterial or an oxide selected from a group consisting of iron oxide,zinc-iron-oxide (Zn—Fe₂O₄ or ZnO—ZnFe₂O₄), phosphor-doped iron oxide,zinc-iron-chromium, bismuth-vanadate, in particular pucheritebismuth-vanadate, vanadium oxide, zirconium-praseodymium-silicate,titanium-antimony-chromium, nickel-antimony-titanium and silver, or themixtures thereof. An inorganic pigment in accordance with the inventionmay be composed of a mixture of a plurality of these materials and/oradditionally contain metallic components. These pigments must, as acondition of use, be temperature resistant up to 900° C.

The object in accordance with the invention is further achieved in thata light system for motorcars that comprises at least one high-pressuredischarge lamp as claimed in claims 1 to 12 is provided.

The high-pressure discharge lamp in accordance with the invention may beused for general lighting purposes. Said high-pressure discharge lampmay particularly be used as a light source in, for example, means oftransport such as aircraft, motor vehicles, motorbikes or the like. Thehigh-pressure discharge lamp in accordance with the invention isparticularly preferably used for headlights, in particular forillumination headlights in motor vehicles such as motorcars.

These and other objects of the invention are apparent from and will beelucidated with reference to the embodiment(s) described hereinafter.

In the drawings:

FIG. 1 shows a CIE 1931 chromaticity diagram,

FIG. 2 shows a high-pressure discharge lamp in accordance with theinvention comprising a burner and an outer bulb,

FIG. 3 shows a diagram of an emission spectrum of a high-pressuredischarge lamp in accordance with the invention.

In FIG. 1, the spectral range in accordance with the invention, whichcorresponds to the ECE R99 standard for “front fog” light, is shown as aplane in the diagram. The color temperatures, also referred to as“correlated color temperature”, i.e. in this case the two lines of equalcolor temperature of 3000 K and 2500 K, respectively, are partlysituated inside this plane. As is shown in the diagram, the colorlocation of the high-pressure lamp in accordance with the invention liesabove the line of the black body radiation.

FIG. 2 shows a high-pressure discharge lamp comprising a burner 2 and anouter bulb 3. Said burner 2, which is customary per se and is madepredominantly of quartz glass comprises a discharge space filled with anionizing gas mixture comprising at least an inert gas and a metal halidemixture containing at least 40 to 80 wt. % NaI and 0 to 40 wt. % ScI. Inthe discharge space, two electrodes with respective electrical contactsare arranged in a customary manner. The burner 2 is attached to thelower end of the tubular outer bulb 3, at least the surfaces of theregions used for attaching the burner 2 to the outer bulb 3 being freeof light-absorbing means and/or interference filters. The outer surfaceof the burner 2 is equipped with a multilayer interference filter 4 thatreflects substantially in the wavelength range of 400 to 550 nm. Theinterference filter 4 has twenty-two layers, the layer structure beingsuch that a layer having a higher refractive index alternates with alayer having a lower refractive index. The eleven layers having thelower refractive index are predominantly composed of SiO₂, and the othereleven layers are composed of zirconium oxide (ZrO₂). The overall layerthickness of the interference filter 4 is approximately 2662 mn.Light-absorbing means 5 having a layer thickness of approximately 850 nmare applied to the inner and the outer surface of the outer bulb 3. Thelight-absorbing means 5 comprises at least Fe₂O₃ pigments which have adiameter of approximately 30 nm and which are integrated in a sol-gelmatrix. The layers of the light-absorbing means 5 can be deposited inknown manner by means of different methods, for example by means ofso-termed PVD or CVD processes, and in the case of light-absorbing means5 with a sol-gel matrix, in particular, by means of spraying or dipcoating.

On the outer surface of the light-absorbing means 5, which is arrangedon the outer surface of the outer bulb 3, there is a multilayerinterference filter 6 which reflects predominantly in the wavelengthrange of 380 to 550 nm, said interference filter 6 covering at leastmost of the surface of the light-absorbing means 5. Said interferencefilter 6 is embodied so as to be multilayered, the multilayer structurebeing such that a layer having a higher refractive index alternates witha layer having a lower refractive index. The layer having the lowerrefractive index is substantially composed of SiO₂, and the second layeris composed of zirconium oxide (ZrO₂), which has a higher refractiveindex than SiO₂. The overall layer thickness of the interference filter4 is approximately 1510 nm, the eight layers of SiO₂ and the eightlayers of ZrO₂ alternating with each other in the layer structure. Thelight transmittance of the interference filter 4 and of the interferencefilter 6 with regard to the wavelength range of 600 to 800 nm is >90%for both, and the light transmittance of the light-absorbing means 5with regard to the wavelength range of 600 to 800 nm ranges between 70and substantially 100%.

The individual layers of the interference filter 4 and of theinterference filter 6 are formed in a customary thin-film process, forexample a so-termed PVD process.

The lamp comprises a customary base 7, so that the lamp can bereplaceably mounted in a front headlight of an automobile.

A light system for automobiles comprising such a high-pressure dischargelamp in accordance with the invention enables a light output ofapproximately 73 lm/Watt to be attained. The color location in the CIE1931 diagram (“chromaticity diagram”) can be sufficiently specified bymeans of the values of both co-ordinates, i.e. X is approximately 0.496and Y is approximately 0.45. The service life of a high-pressuredischarge lamp in accordance with the invention is at least 1000 hours.

FIG. 3 shows the diagram of an emission spectrum of the high-pressuredischarge lamp in accordance with the invention as shown in FIG. 2.

1. A high-pressure discharge lamp (1) comprising at least one burner (2)having a discharge space, two electrodes extending in the dischargespace, a gas filling in the discharge space that contains at least aninert gas and a metal halide mixture, and comprising a tubular outerbulb (3) having two ends, the burner (2) being attached, at least at oneend, to the outer bulb (3), characterized in that the outer bulb (3)comprises at least one light-absorbing means (5) and at least oneinterference filter (6), and an interference filter (4) is arranged onor in at least a part of the burner (2).
 2. A high-pressure dischargelamp (1) as claimed in claim 1, characterized in that the interferencefilter (4) is arranged on the outer surface of the burner.
 3. Ahigh-pressure discharge lamp (1) as claimed in claim 1, characterized inthat a light-absorbing means (5) is provided on the inner surface of theouter bulb (3), and a light-absorbing means (5) is provided between theouter surface of the outer bulb (3) and the interference filter (6). 4.A high-pressure discharge lamp (1) as claimed in claim 1, characterizedin that at least at the surfaces of the areas that are used to attachthe burner (2) to the outer bulb (3), no light-absorbing means and/orinterference filters are arranged.
 5. A high-pressure discharge lamp (1)as claimed in claim 1, characterized in that the light transmittance ofthe interference filter (4) and of the interference filter (6), withregard to the wavelength range of 600 to 800 nm, is >90% for both.
 6. Ahigh-pressure discharge lamp (1) as claimed in claim 1, characterized inthat the light transmittance of the light-absorbing means (5) withregard to the wavelength range of 600 to 800 nm ranges between 70 andsubstantially 100%.
 7. A high-pressure discharge lamp (1) as claimed inclaim 1, characterized in that the layer thickness of the interferencefilters ranges between 800 and 2800 nm.
 8. A high-pressure dischargelamp (1) as claimed in claim 1, characterized in that the interferencefilter is composed of a plurality of layers, the layer structure beingsuch that a layer having a higher refractive index alternates with alayer having a lower refractive index, the layer having the lowerrefractive index preferably consisting predominantly of SiO₂ and thesecond layer being composed of a material having a higher refractiveindex than SiO₂.
 9. A high-pressure discharge lamp (1) as claimed inclaim 8, characterized in that the second layer is composed of amaterial selected from the group consisting of titanium oxide, tantalumoxide, niobium oxide, hafnium oxide, silicium nitride, very preferablyzirconium oxide ZrO₂, or a mixture of said materials.
 10. Ahigh-pressure discharge lamp (1) as claimed in claim 1, characterized inthat the layer thickness of the light-absorbing means (5) ranges between5 nm and 10,000 nm.
 11. A high-pressure discharge lamp (1) as claimed inclaim 1, characterized in that the light-absorbing means (5) containsinorganic pigments, which absorb part of the visible light and theaverage diameter of which is below 100 nm.
 12. A high-pressure dischargelamp (1) as claimed in claim 11, characterized in that the inorganicpigment is composed of a material or an oxide selected from a groupconsisting of iron oxide, zinc-iron-oxide (Zn—Fe₂O₄ or ZnO—ZnFe₂O₄),phosphor-doped iron oxide, zinc-iron-chromium, bismuth-vanadate, inparticular pucherite bismuth-vanadate, vanadium oxide,zirconium-praseodymium-silicate, titanium-antimony-chromium,nickel-antimony-titanium and silver, or the mixtures thereof.
 13. Alight system for motorcars comprising at least a high-pressure dischargelamp (1) as claimed in claims 1 through 12.